I. Abstract of the Paper’s Core Research

This study innovatively integrates a ring‑shaped piezoelectric acoustic module with an aerosol‑jet printing head, establishes a multiphysics coupled simulation model of the airflow–acoustic field, and optimizes the optimal acoustic focusing frequency at 5.8 MHz. The system then systematically compares the line morphology, electrical conductivity, and long-term stability between conventional pneumatic printing and acoustically enhanced printing. The key findings are as follows:

Core Mechanism of Radial Beamforming in a Circular Acoustic Field

The inverse piezoelectric effect in the annular piezoelectric tube generates an annular ultrasonic field, producing radial acoustic radiation forces that continuously propel aerosol nanoparticles toward the nozzle’s central axis. The spray beam width is reduced to 60% of its original value, and beam convergence or divergence can be switched reversibly within milliseconds without adjusting the airflow parameters.

Breaking the performance limits of micron-scale printing

In the conventional pneumatic process, the minimum line width is 9.68 μm and the overflow is 0.66 μm; under acoustically enhanced conditions, the optimal operating state yields a line width of 5.71 μm with an overflow of only 0.08 μm, corresponding to an overflow ratio of less than 2%. Furthermore, it is possible to mass‑produce 24‑channel high‑density serpentine electrodes with both line width and line spacing at 6 μm, free of short‑circuit defects.

The thin film becomes denser, resulting in a significant enhancement of its electrical conductivity.

Acoustic confinement enables tighter stacking of silver nanoparticles, reducing the film’s porosity from 22.6% to 3.4%; at the same line length, the electrical resistance decreases by 35%, and the electrical conductivity increases by approximately 180%, without altering the original grain size of the nanoparticles.

Ultra-high stability for long-term continuous printing

6-μm ultra-fine traces can be stably and continuously printed for 4.5 hours, while standard 10/20/50-μm traces operate reliably for 8 hours. Line-width variation is maintained within ±0.3 μm, and the optimal nozzle‑to‑substrate distance has been extended to 1–6 mm.

Generalized Acoustic Enhancement Printing Scheme

Acoustic focusing is compatible with silver-based nano‑inks, and the printhead architecture supports a wide range of metal and composite functional pastes; high‑precision conformal wiring can be achieved on planar, curved, and irregularly shaped substrates.

II. Complete Preparation Procedure for Aerosol Enhancement in a Ring-Shaped Acoustic Field

1) Ultrasonic dispersion of 35 nm silver nanoparticle conductive ink to eliminate particle agglomeration;

2) An improved dual-zone pneumatic–acoustic composite print nozzle, featuring a built-in annular piezoelectric ultrasonic actuator;

3) The equipment generates aerosol droplets via ultrasonic atomization, and a nitrogen carrier gas transports them to the acoustic zone of the nozzle.

4) Activate the optimal 5.8 MHz annular acoustic field, enabling radial convergence of acoustic radiation force to focus the acoustic beam and perform point‑by‑point direct writing of micro‑circuits.

5) Sintering and shaping are performed uniformly in a 200°C constant-temperature oven.

6) Optical profilometers, SEM, and four-point probe instruments are used to characterize line dimensions, porosity, and electrical conductivity.

III. Core Advantages of the Three- to Five‑Layer Aerosol Acoustic Composite Process (Compared with Conventional Pneumatic AJP)

Sub-6μm ultra-high printing resolution, with virtually no material overflow.

By relying solely on acoustic field focusing, ultra-fine lines can be achieved without reducing the nozzle aperture; the edges exhibit no silver‑powder smearing, and the high‑density microarray avoids short circuits.

Simultaneously enhance both the line density and electrical conductivity.

The acoustic field compresses the particles into a tightly packed structure, reducing internal porosity and significantly enhancing electrical conductivity without the need to modify the sintering process.

Millisecond-level reversible focusing, with on-the-fly switching between thick and thin lines.

The acoustic field switch enables rapid control of beam width, allowing simultaneous fabrication of thick lead wires and ultra‑fine sensing electrodes within a single printing process.

The spray distance has a broader compatibility range, making it suitable for irregularly shaped curved-surface components.

The effective print nozzle-to-substrate distance has been extended from 5 mm to 6 mm, while high precision is still maintained on both concave/convex and curved conformal substrates.

The ink is highly versatile and easy to modify.

The acoustic module can be directly retrofitted to existing commercial aerosol equipment, and is compatible with both metallic and carbon-based functional inks.

IV. Five Core Technological Innovations of This Study

Innovation 1: Integrated Annular Radial Acoustic Focusing Aerosol Nozzle

Unlike single-sided ultrasonic focusing schemes, the annular acoustic field achieves 360° omnidirectional simultaneous droplet convergence, with a uniform beam and no unilateral deviation.

Innovative Multi-Physics Coupled Deposition Prediction Model

By simultaneously solving the acoustic radiation force and fluid drag equations, we simulate particle trajectories, accurately predict line width and overflow levels, and significantly reduce process trial-and-error.

Innovation: A 3.58 MHz standardized optimal acoustic frequency system

Quantitatively match the nozzle inner diameter to the ultrasonic wavelength, determine a dedicated focusing frequency for silver nanoparticle ink, and establish a reusable set of process parameters.

Innovative 4‑wire, short‑circuit‑free, 6‑μm pitch, high‑density microarray fabrication process

It addresses the longstanding challenge of bridging and short-circuiting in fine-line spacing inherent to conventional processes, enabling the mass production of highly integrated MEMS devices and flexible sensor arrays.

Innovation: A Complete Solution for Long-Term Stable Ultra-Fine Printing

Overcoming the challenge of poor print‑time consistency in ultra‑fine‑line printing, it achieves highly stable dimensional accuracy over several hours of continuous build, meeting the stability requirements of mass‑production processes.

V. Current Industry Challenges and Corresponding Solutions

Challenge 1: In conventional pneumatic aerosol beam delivery, beam divergence is severe, leading to significant material spillage and a high risk of short circuits in high-density circuitry.

✅ The annular sound field radially converges mist droplets over 360°, reducing aerosol particle size to as low as 0.08 μm, and the 6-μm ultra-fine pitch array exhibits no connectivity defects.

Challenge 2: After printing ultra-fine lines, the resulting film is porous, leading to significant conductive losses.

✅ Acoustic radiation force compresses particle aggregates, significantly reducing porosity and nearly doubling the conductive performance at the same wiring density.

Challenge 3: Line thickness can only be adjusted via airflow, resulting in slow switching and complex parameter coupling.

✅ Independent control of the sound field, with millisecond‑level switching of beam width and no need to repeatedly adjust airflow parameters.

Challenge 4: Ultra-fine printing exhibits short process‑stability times and poor mass‑production consistency.

✅ Acoustic confinement stabilizes the mist beam, enabling continuous printing of 6-μm‑wide lines for 4.5 hours and conventional traces for 8 hours, with minimal dimensional variation.

Challenge 5: Existing high-precision solutions rely on extremely small nozzles, which are prone to clogging and result in high consumable costs.

✅ A large-aperture nozzle paired with acoustic beam focusing delivers both smooth printing and ultra-high resolution below 6 μm.

VI. Future Diversified Industrial Application Scenarios

1 Micro‑MEMS sensor with high‑density interdigitated electrodes

Pressure, temperature sensing, and bio‑microfluidic chips, coupled with micron‑scale precision sensor arrays, enhance detection sensitivity.

2 Foldable-screen ultra-thin micro-drive wiring

Flexible displays feature highly integrated pixel circuits and narrow pitch with no material overflow, reducing the rate of screen short‑circuit scrap.

3 Implantable Flexible Brain–Computer Interface Electrodes

Ultra‑fine, highly conductive wiring achieves lightweight, conformal adhesion to brain tissue, enhancing the precision of neural signal acquisition.

4 Miniature RF Passive Components (Inductors, Resistors)

High-precision micro-metallic structures reduce the footprint of RF chips, enabling compatibility with wearable miniature communication devices.

5 Laboratory-Based Rapid R&D Platform for Micro- and Nanoelectronics

By retrofitting existing aerosol equipment with an acoustic module, high‑precision sample preparation down to sub‑10 μm can be achieved at low cost.

Image and Text Guide

Figure 1. Overall concept, nozzle design, and focusing mechanism of annular acoustic-field-assisted AJP (AF-AJP)

Figure 1(a) shows an annular piezoelectric ceramic tube fitted over the outer wall of the original AJP nozzle, driven by a signal generator and a power amplifier; Figure 1(b) depicts the modified nozzle, which incorporates two sections of pneumatic focusing cones flanking a cylindrical section of piezoelectric tube; Figure 1(c) illustrates that the initial liquid‑jet diameter D₀ is radially constricted to D_A under the influence of the annular acoustic field. Figure 1(d) presents a continuous printing line with three switching states—AF OFF→ON→OFF—clearly demonstrating that the jet narrows immediately upon AF activation and recovers instantly upon deactivation. Figures 1(e/f) provide top‑view and cross‑sectional perspectives: the annular acoustic field focuses aerosol particles toward the central axis; Figure 1(g) shows the force balance on a single particle—axial Stokes drag pushes the particle downward, while radial acoustic radiation force drives it toward the center, with Saffman lift and gravity acting in superposition.

Figure 2. Effect of AF on the geometric morphology of AJP lines

Figure 2(a): AF OFF—linewidth LW = 9.68 μm, overspray 0.66 μm; Figure 2(b): AF ON—LW narrows to 5.71 μm, overspray is reduced to 0.08 μm; Figure 2(c): After AF is turned off, LW returns to 9.77 μm and overspray resumes at 0.72 μm—indicating a fully reversible process. Figure 2(d): On a single continuous line, three consecutive AF OFF→ON→OFF transitions are shown, with the linewidth LW changing instantaneously within the red box. Figures 2(e/f): High-magnification topography: under AF ON, particles converge toward the center; when AF is turned off, particles diverge. The length of the transition region l_p ≈ 8.2 μm, corresponding to a transition time of ≈0.9 ms.

Figure 3. Multi‑width repeatability, omnidirectional focusing, and high‑density array verification

Figure 3(a–d): Gaussian cross-sectional profiles of the three AF states—AF OFF1, AF ON, and AF OFF2—along with optical microscopy images, for four target line widths (10/30/50/80 μm). Under AF ON, the peak becomes sharper and taller, with a narrower base; AF OFF2 nearly overlaps AF OFF1, demonstrating reversibility. Figure 3(e): Two-dimensional orthogonal‑direction helical printing with intermittent switching between AF ON and AF OFF; at five test points, the I–V characteristics exhibit uniform line widths, confirming omnidirectional X/Y focusing. Figure 3(f): A high‑density serpentine array comprising 24 lines, with a line width of 6 ± 0.3 μm, a pitch of 6 ± 0.3 μm, and an overprint of 0.3 ± 0.1 μm, showing no short circuits. Figure 3(g): On a single continuous line, six consecutive AF ON/OFF transitions result in instantaneous line‑width variations ranging from 10.67 to 6.93 to 10.96 to 7.02 to 11.15 to 7.26 μm.

Figure 4. Effect of AF on electrical properties and internal microstructure

Figure 4(a) shows the AF OFF/ON resistance under identical lengths but varying line widths: AF ON reduces the resistance by 35±5% (indicated by the cyan trend curve). Figure 4(b) illustrates that the conductivity increases from ~5.5×10⁶ S/m in AF OFF to ~10×10⁶ S/m in AF ON, corresponding to a 180±5% enhancement. Figures 4(c–e) present surface SEM images: in AF OFF, the structure is highly porous and loosely organized; in AF ON, particles are densely packed; and in AF OFF2, porosity reappears. Figures 4(f–h) show cross-sectional SEM images: the porosity of the AF ON cross-section decreases from 22.6±1.3% to 3.4±0.8%, reverting to 21.7±0.9% upon turning off the AF. Figure 4(i) provides a schematic of the underlying mechanism: PVP‑wrapped Ag nanoparticles are driven toward the center by acoustic radiation forces, leading to denser particle packing within the droplet and, following sintering, more continuous conductive pathways.

Figure 5. Comparison of 8-hour continuous printing stability with literature data.

Figure 5(a/b): At four resolution settings (6, 10, 20, and 50 μm), line width and thickness were recorded every 20 minutes over an 8-hour period. The 10-, 20-, and 50-μm settings remained stable throughout the entire duration, whereas the 6-μm setting stabilized for only 4.5 hours. The average line widths were 6.0 ± 0.3, 10.8 ± 1.2, 20.6 ± 1.4, and 50.7 ± 1.4 μm, respectively. Figure 5(c) presents a horizontal comparison with ten previously published AJP‑optimized studies: the x‑axis shows resolution in μm, and the color bars indicate the quality of overspray (yellow = good; purple = poor). In most prior work, the optimal resolution was typically 15–30 μm, with noticeable overspray; by contrast, this study (yellow bar) achieves a resolution of approximately 6 μm and an overspray of less than 0.1 μm, making it the first approach to simultaneously outperform existing methods in both resolution and overspray control.